Increasing the filler content of paper can provide the papermaker with numerous benefits, including savings in the cost of raw materials, improved optical properties, and better print quality. There are, however, limits to the amount of filler that can be substituted for papermaking fiber. At high filler contents, paper can suffer losses in strength, stiffness and sizing. All commonly used untreated fillers (e.g. clay, titanium dioxide, calcium carbonate) are known to have a detrimental effect on strength and sizing. Increasing the concentration of filler in conventional paper-making furnishes results in increased size demand to maintain the desired degree of water repellency in the finished paper. This is because a disproportionate fraction of the size is adsorbed on the high surface area filler in the furnish. This adsorbed size may be lost from the furnish due to poor retention, or more likely, the increased sizing demand is due to the manner in which the size becomes attached on the conventional high surface area filler components. This link is not permanent and does not contribute significantly to the paper's hydrophobicity. Thus the effectiveness of the size is reduced which results in an increase in sizing demand (see article entitled "Diagnostic Sizing Loss Problem Solving in Alkaline Systems", B. M. Moyers, 1991 TAPPI Papermakers Conference Proceedings, pages 425-432).
In particular, poor sizing efficiency and loss of water repellency over time (size reversion) are problems associated with the use of alkyl ketene dimer (AKD) and alkenyl succinic anhydride (ASA) sizing agents and calcium carbonate pigments, particularly in highly filled alkaline papers. These problems are generally accentuated when filler levels approach and exceed about 20%. In addition, strength properties decrease as filler levels increase, impacting negatively on converting operations and end use functionality. Thus, in circumstances where increasing filler content would be advantageous, associated sizing problems have occurred affecting paper quality, machine performance and runnability.
The mechanism by which permanent AKD sizing is imparted to alkaline papers is generally agreed to include the retention, distribution and anchoring of the size with proper molecular orientation onto the cellulose fibers. There has been some controversy as to the anchoring mechanism itself and whether or not a covalent bond is formed between cellulose hydroxyl groups and the lactone ring of the AKD molecule. Sizing losses over time have been attributed to interference on the molecular level by wet end additives and contaminants, as well as AKD/pigment interactions and the effects of AKD hydrolysis products. These results are reported for instance in the article entitled "The Interactions of Alkyl Ketene Dimer with other Wet-End Additives", by A. R. Colasurdo and I. Thorn, TAPPI Journal, September 1992, pages 143-149.
AKD sizing agents are basically waxes with relatively low melting points (approximately 115.degree. F.). Their fluidity increases as sheet drying temperatures increase. According to accepted theory, once the size is retained in the sheet matrix, the sub-micron particles melt and spread over fiber and filler surfaces during the drying process before reacting with the cellulose fibers.
Efforts by others to reduce the sizing demand of filled papers include the invention disclosed in U.S. Pat. No. 5,147,507. This patent teaches a method for improving papermaking with the use of a calcium carbonate pigment modified by surface treatment with a cationic polymer. A polymer of the type used is sold under the tradename HERCON. In fact, HERCON is a tradename for a reactive size material. Thus, the '507 patent teaches a process whereby a reactive size is associated directly with the calcium carbonate pigment. The result is alleged to improve the papermaking process by reducing the usage of wet end sizing agent, improving opacity, improving filler retention and causing better drainage on the papermachine. However when used according to the patent, the surface treated pigment is combined with synthetic sizing agents such as AKD or ASA which results in the use of more sizing agent than would be required according to the present invention. In addition, the treatment specified in the '507 patent is more expensive than the process of the present invention.